Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Quantum interpretations

  1. Jan 16, 2009 #1
    I was gassing away on another forum about how quantum mechanics was non-local and so on, and someone said he didn't believe it was, and that other interpretations with local or hidden variables were more valid, this seems converse to what I'm learning. Consider me a Student of physics with a early degree level of the maths, so please don't go too heavy. Can anyone explain why he's mistaken? It seems to me interpretations like the Copenhagen interpretation are more mainstream?
  2. jcsd
  3. Jan 16, 2009 #2
    Einstein didn't like QM very much. One of the cornerstones of relativity is that all frames were created equal. It has been known since Gallileo that one object's "stopped" is another object's "moving." Einstein's relativity took it a step further and accounted for the rate of the passage of time as well. A clock in a rocket ticks slower than the watch on your wrist. But to a passenger on the rocket, it is your watch that is moving slowly. One object's "proper" time is another object's "slowed" (or "dilated") time.

    The main argument for the validity of relativity is based on the speed of light. It's constant to every observer. That means if you are at rest, it moves at c. If a train moving at 80mph sees the same beam light, it will measure it's speed as c and NOT c + 80mph as should be expected by classical theory. Time slows down to compensate.

    Now time slowing down seems paradoxical, but as long as nothing moves faster than c, everything's dandy. That means no information can be sent faster than c. No gravity or electric fields and especially not matter. If you can travel faster than c, suddenly you have contradicted yourself, because you can send a signal to another rocket and say "I have irrefutable evidence that YOU are the one who is slow." And the world is forced to distinguish between "at rest" and "moving."

    So that's why light can't move faster than c.

    The problem is that QM DOES allow things to move faster than c. But only in special ways. When you entangle two particles, they share a single quantum state. The particles can be split up physically in space. When one particle is measured, the state collapses for both particles. Suppose we have two particles in the state 1/sqrt 2 |+-> + 1/sqrt 2 |-+> That is, when you measure the particle, there is a 50% chance the first particle will be spin up and the second, spin down and a 50% chance where you get the reverse. When you measure one particle, the state collapses, and both particles have opposite spins. That means that the result of the measurement must travel to the other particle instantaneously to let it know what the result is. The only reason this kind of instantaneous influence is allowed is that the results of the measurement are random. Even though the influence is infinitely fast, there is no way to send any messages through it. Similarly, there is no way to tell using one particle to tell if the other particle has been measured yet. Flow of time is still, as far as either party can tell, entirely relative.

    There is an important thought-experiment (which I'm not 100% clear on) called the Bell Theorem that deals with this issue.
  4. Jan 16, 2009 #3
    Thanks that's a great answer.

    I am aware of Bell's it states that:

    "No physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics."

    The thought experiment is valid, the experiment shows that indeed according to probability QM is non-local.

    I'm wondering why someone would try to claim hidden local variable or non local hidden are more valid? The guy is very educated but I can't understand why he believes that the Copenhagen theory that is probabilistic is wrong and it is deterministic ie wave function real, hidden variable/local instead.

    I know about Einstein and the EPR paradox, but this is debunked by Bell's? So why would someone with a post doctoral education challenge that issue and how?

    Bell's also implies that no experiment however non-local can transfer information faster than c. But this is slightly off track.
  5. Jan 16, 2009 #4
    Check Multi-Worlds. It is the best interpretation
    Once you accept multi-worlds, there are no more paradoxes.
    No weird stuff like 'wavefunction collapse' etc
    It is even deterministic (God does not play dice!!!)
  6. Jan 16, 2009 #5
    You mean many worlds, yes I know about it in reading about interpretations, but it is not as favoured as CI, what evidence is there that MWI may be true? And if it is how would we show it in experiment?

    God does not play dice? :smile:

    But didn't Bohr say stop telling God what to do with his dice?

    I googled it I must admit. :biggrin:
  7. Jan 16, 2009 #6
  8. Jan 16, 2009 #7
    Thanks for that I read it, but I don't see how this provides evidence, it's all mathematical and light on experiment? Maybe I missed something...

    Kepler Newton>classical physics>landscape plus inflation>level IV multiverse>fundamental laws.

    Isn't that a set of non sequiturs?

    :confused: eh?
  9. Jan 16, 2009 #8
    Interpretations can not be proven experimentally (otherwise they would not be called interpretations). They are all consistent with QM (except local hidden variables and objective collapse theories)

    Max just gives many arguments on the phylosophycal level.

    MWI is just CI *without* wavefunction collapse. So if we can describe everything without a collapse, shouldnt the collapse be cut but the Occam'z razor?

    We can not see alternative universes, but he wrotes
    "The key point to remember is that parallel universes are not a theory, but a prediction of certain theories. For a theory to be falsifiable, we need not be able to observe and test all its predictions, merely at least one of them"

    Then he compares parralel branches to the interiour of the black holes (we can not observe them but we speculate a lot about what is inside, so modern physics had already abandoned the falsiability)
  10. Jan 16, 2009 #9
    Ah yeah, good point.

    Can I point out thought that Occam's razor is not a law of science it's a suggestion. Is MWI discrete from CI, if so in what ways, and can we show that those ways are enough to make it theoretical?

    Speculation on speculation?

    We infer black holes by gravity and the maths, trying to say what would happen given the maths seems a bit cart <horse, before>, if you see what I mean.
  11. Jan 16, 2009 #10
    I quess the most important difference between CI and MWI is that MWI is deterministic. So we have a hope to finally get TOE equations in a form TOE(fi)=0, in a pure mathematical form.

    Withj the probability stuff there is an inavoidable 'blah-blah-blah' part. For example, there are 2 different interpretations of what a probability is
  12. Jan 16, 2009 #11
    Regarding CI, And I forgot about the mystical role of some QM systems, called "measurement systems" magically affecting the wavefunctions... I guess this is a weirdest part of CI
  13. Jan 16, 2009 #12
    It's not magical it just is. Science of course has no more regard for your problems of understanding than the Universe cares about the way you are and and how you interpret it, it just is. Perhaps we should just say shut up and speculate?
  14. Jan 16, 2009 #13
    And yet Bell's say is it isn't? How now brown cow?

    Deterministic equals real wave function, hidden variables, non-local and local either or.

    Probabilistic equals non-local no hidden variables. There's the best definition I think.
  15. Jan 16, 2009 #14
    Wait, wait.
    Deterministic = complex :) valued wave function which has a REAL (physical) meaning.
    But no hidden variables (except the wavefunction itself)
    either??? where and why???
  16. Jan 16, 2009 #15
    For the pure mathematical reasons I do believe that the definition(axiomatisation) of QM *should not* include any references to the huge ensembles of the molecules called 'observers' and 'measurement devices'

    Otherwise there is a bad (infinite) recursion.

    You ought to define QM without any references to a 'measurement'.
  17. Jan 16, 2009 #16
    (This from an amateur testing his chops) I think your question comes down to, “what’s a clear argument I can make against hidden variables?” And if your protagonist is familiar with Bell’s theorem, then he must have an argument for that. I can’t imagine what that is since the matter seams to be settled. So maybe it has something to do with the locality of those hidden variables. So I’ll mention the Bohmian interpretation, which has advantages like many worlds in that it can simplify things. It’s considered to be a hidden variable interpretation but those variables are also non-local. The Bohmian interpretation is considered to be legitimate although it’s still just an interpretation. There are other interpretations containing non-local hidden variables. (See http://en.wikipedia.org/wiki/Hidden_variable_theory" [Broken])
    Last edited by a moderator: May 3, 2017
  18. Jan 16, 2009 #17


    User Avatar
    Science Advisor
    Gold Member

    Correct that Occam's Razor is a suggestion... each of us can reasonably adopt - in the best scientific tradition - any of several equivalent interpretations. CI (sometimes called "shut up and calculate") is popular, Many Worlds is popular, and Bohmian Mechanics (a/k/a dBB) is popular. There are more, there is the Cramer's (I think) Transactional Interpretation, Relational Interpretation, and one I think has merit (at least enough to look at) called Time Symmetric QM.

    Pick one from column A, two from column B... it's really a matter of personal preference although some insist that one interpretation's virtues are greater than another's.
  19. Jan 16, 2009 #18
    Sorry I was confused a little bit
    Correct, MWI contains non-local hidden variables - but only for an observer (called 'frog's view')
    In the 'bird's view' (the view of God, where we observe all branches of reality) there are no hidden variables.
  20. Jan 16, 2009 #19
    Is it like the Transactional interpretation?
  21. Jan 16, 2009 #20


    User Avatar
    Science Advisor
    Gold Member

    In the Time Symmetric view, the future can influence the present/past. Relativity is retained, realism is dropped. So a future measurement context influences the present, and this provides a mechanical explanation that is intended to remove the time asymmetry implied by wave function collapse.

    I'm not selling this baby, too new to see all of the ramifications yet, but I think it is promising. I also think it is an intuitive way to visualize what happens. There may be similarities to TI, I will have to look at it again.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook